8142

• UCREFRP Catalog Number: 8142
• Author: Horn, M. J.
• Title: Nutritional Limitation of Recruitment in the Razorback Sucker ‎(
• USFW Year: 1996.
• Copyright Material: NO

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of individual plasticity (Sousa 1984; Poffand Ward 1990; Schlosser 1991).

Due to high commercial and recreational value, some of the best documented and

studied examples of species' decline resulting from human-induced impacts are in fishes

such as salmons (Netboy 1974; Clay 1995). Overfishing, habitat loss or modification,

and introduced species all are implicated. Often more than one factor may come into play

at once, making it difficult to determine which is more important (Karr and Dudley

1989). More than a billion dollars have been spent to restore Columbia River salmon

fisheries yet the populations are rapidly approaching non-viable levels (Nehlsen et al.

1991). The problem, however, extends far beyond commerce and recreation. There are

few if any major rivers worldwide where one to many indigenous species are not

imperiled. All larger north temperate and tropical streams and rivers have been modified

(Sparks et al. 1990; Dynesius and Nilsson 1994; We1comme 1995). As of 1976 there

were 1426 hydropower and 27 pump-storage facilities and many smaller facilities

damming rivers and streams in the United States alone (USFPRC 1976; USFWS 1976).

The high human demand for water, particularly in arid lands, amplifies declines in aquatic

ecosystems (Walker et al. 1995). Vast changes in biotic structure, including species' and

community extinctions, have occurred and are under way (Miller 1961; Minckley and

Deacon 1968, 1991; Minckley 1991; Minckley and Douglas 1991).

For fishes in general, declining populations in altered aquatic habitats most often

result from shifts in the pattern of survivorship among younger cohorts (Houde 1987).

Declining abundance, particularly in fishes, provides a good subject for study because

extrinsic, density independent factors typically affect early survivorship (Lawler 1965;

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